Cell therapy trials suggest that bone marrow (BM)-derived progenitor cells (PCs) could be a promising new option for treating ischemic heart disease, but the efficacy of this approach has been modest. Some of the barriers to successful cell therapy include poor recruitment of cells from the BM to the site of injury and the low proportion of recruited cells that are retained and survive in the ischemic region. Both the release (i.e., mobilization) of PCs from the BM to the peripheral blood and the recruitment and retention of PCs in ischemic tissue are regulated by interactions between stromal cell-derived factor-1 (SDF-1) and CXC chemokine receptor 4 (CXCR4), and a recent publication from our laboratory shows that in the BM, SDF-1/CXCR4 signaling is linked to c-kit activity. C-kit is a type III receptor tyrosine kinase and is expressed on the surface of many types of PCs, where it functions as a receptor for stem cell factor (SCF) and is crucial for BM PC- mediated cardiac repair. Our published observations indicate that SDF-1/CXCR4 signaling induces c-kit phosphorylation (transactivation), which retains PCs, in the BM, and that the antagonism or genetic deletion of CXCR4 mobilizes PCs by downregulating SDF-1-mediated (but not SCF-mediated) c-kit activity. We also showed that SDF-1-induced c-kit phosphorylation requires activation of the Src family of non-receptor tyrosine kinases (SFKs)-SFK inhibition blocked both c-kit phosphorylation and the interaction between c-kit and phosphorylated Src in BM PCs-and our ongoing experiments indicate that c-kit-deficiencies impair the migration of BM PCs toward SDF-1 and significantly reduce BM PC recruitment to the ischemic myocardium. The goal of this application is to establish the role of SDF-1/CXCR4-c-kit signaling in BM PC recruitment, and to determine whether techniques that target this pathway could enhance the effectiveness of cell therapy. Our central hypothesis is that SDF-1/CXCR4 signaling activates (i.e., phosphorylates) c-kit, and that c-kit activation subsequently leads to the recruitment and retention of PCs in the ischemic region; thus, treatments that enhance c-kit activity in circulating PCs will improve PC recruitment and PC-mediated cardiac repair by increasing the sensitivity of PCs to SDF-1/CXCR4 signaling. We will accomplish our objective by evaluating our central hypothesis with a series of experiments grouped under three specific aims: Aim 1) to characterize the molecules and mechanisms involved in SDF-1/CXCR4-c-kit signaling; Aim 2) to identify the role of SDF- 1/CXCR4-c-kit signaling in BM PC recruitment; and Aim 3) to determine whether PC recruitment and cardiac repair after MI can be improved by increasing c-kit activity in BM PCs. We anticipate that the work proposed in this project will provide novel insights into the mechanism by which the SDF-1/CXCR4-c-kit pathway regulates BM PC recruitment and retention in ischemic tissue, thereby identifying new therapeutic approaches and targets that could enhance the effectiveness of cell therapy for treatment of ischemic heart disease.